In the field of electronics, ceramics are used in a variety of applications, including hybrid circuit substrates, passive components, semiconductor packages, and multilayer substrates. Multilayer substrates are formed by laminating a plurality of thin ceramic layers together. Conductive patterns are formed on some or all of the layers prior to lamination, and are selectively connected together in the laminated structure through openings, or "vias", in the individual layers. The ceramic compositions used in such applications generally require very high temperatures (e.g., 1600.degree. C.) for sintering. As a result, the conductive patterns in multilayer ceramic circuits must be formed of refractory metals, such as tungsten, molybdenum, or molybdenum/manganese ("molymanganese"), since metals conventionally used for electronic circuits--copper, gold, silver, and aluminum--would melt during the sintering operation.
The industry has sought in recent years to develop a glass-ceramic material that can be sintered at lower temperatures. Among other advantages, this would permit low resistance, nonrefractory metals, such as gold, silver, and copper, to be used for circuit conductors. The efforts to develop lower firing temperature ceramics have also been directed at obtaining compositions that may be fired in nonoxidizing atmospheres, such as nitrogen, forming gas, or hydrogen, as well as in air, to permit copper to be used as a conductor material. Conventional air firing would oxidize circuitry formed of copper.
Low firing temperature multilayer glass-ceramic substrates have been manufactured from a composition containing 55 wt.% alumina and 45 wt.% lead borosilicate. The environmental toxicity of lead is well documented, and it is therefore desirable to avoid the use of lead compounds in such compositions. Multilayer ceramic substrates also have been produced from a mixture of Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, and MgO sintered at 800.degree. to 1,000.degree. C. in air. The coefficient of thermal expansion of the resulting ceramic is somewhat higher than desired, thus reducing the structural stability of the substrate.